U.S. patent number 5,311,116 [Application Number 07/862,621] was granted by the patent office on 1994-05-10 for multi-channel electromagnetically transparent voltage waveform monitor link.
This patent grant is currently assigned to Electronic Development, Inc.. Invention is credited to Wesley A. Rogers.
United States Patent |
5,311,116 |
Rogers |
May 10, 1994 |
Multi-channel electromagnetically transparent voltage waveform
monitor link
Abstract
A multi-channel electromagnetically transparent voltage probe
transmission link system for monitoring a plurality of voltage
signals at a plurality of test points of a device under test that
is subjected to a radiation field. Each channel includes two
voltage probes, an electrical to optical signal transmitter, an
optical signal transmission line and a receiver located out of the
radiation field. The voltage probes contact and sense the voltage
signals at the test point. The electrical to optical transmitters
are removably mounted in a common base and are powered by either a
common (or shared) power supply and/or by dedicated power supplies,
such as rechargeable batteries. The receivers process the optical
signals and provide display signals corresponding to the sensed
voltage signal at the plurality of test points for evaluating the
effect of the test radiation field. An attenuator which may be
electromagnetically transparent is provided for attenuating a
sensed voltage signal to a range suitable for processing by a
transmitter device having a limited input range, over a frequency
range of interest. The input circuits to the transmitter are
provided with a centertap which provides a return current path for
the voltage signals of the device under test that are sensed by a
voltage probe. Low power consumption and low voltage drift circuits
are used.
Inventors: |
Rogers; Wesley A. (Grosse
Pointe Park, MI) |
Assignee: |
Electronic Development, Inc.
(Grosse Pointe Park, MI)
|
Family
ID: |
25338875 |
Appl.
No.: |
07/862,621 |
Filed: |
April 2, 1992 |
Current U.S.
Class: |
324/72.5;
324/627; 324/72 |
Current CPC
Class: |
G01R
31/002 (20130101); G01R 29/0821 (20130101) |
Current International
Class: |
G01R
31/00 (20060101); G01R 031/02 () |
Field of
Search: |
;324/627,628,158P,72,72.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Knowles et al. "Cable Shielding Effectiveness Testing" IEEE Trans.
on Electromagnetic Compatibility, vol. EMC-16, No. 1, pp. 16-23,
Feb. 1974. .
Preliminary Product Bulletin for Flurosint.sup.R 719 Insullated
Monofilament and Flurosint.sup.R 819 Dual Line, by the Polymer
Corporation, Feb., 1987. .
Vehicle Level EMC Testing Methodology, by Gary F. E. Vrooman, Ford
Motor Company, Jan., 1988. .
Bulletin for Ailtech Current Probes, by Eaton Advanced Electronic,
Nov., 1981. .
Product Literature, Broadband Isotropic Probe Systems, by EMCO,
Jan., 1990. .
Product Literature, Model 5188 and Model 3718 Micrograbber
Testclips, by ITT Pomona Electronics, Jan. 1989..
|
Primary Examiner: Wieder; Kenneth A.
Assistant Examiner: Brown; Glenn
Attorney, Agent or Firm: Davis Hoxie Faithfull &
Hapgood
Claims
I claim:
1. Apparatus for monitoring the effect of a radiation field at a
plurality of test points of a device under test comprising:
a plurality of voltage probe transmission links, each link
comprising:
a voltage probe for contacting and sensing a first voltage signal
at one test point;
a converter circuit for converting the first sensed voltage signal
to a corresponding optical signal; and
a common power supply and a plurality of cables for providing power
to said plurality of converter circuits, wherein said common power
supply and said plurality of converter circuits are radiation
hardened and said voltage probe is transparent to the radiation
field so that the apparatus does not inject signals in any of the
test points of the device.
2. The apparatus of claim I further comprising a plurality of power
supplies such that there also is one dedicated power supply for
each converter circuit.
3. The apparatus of claim 2 wherein each converter circuit and its
associated dedicated power supply are enclosed in a radiation
hardened enclosure.
4. The apparatus of claim 2 wherein each dedicated power supply is
a rechargeable battery.
5. The apparatus of claim 4 wherein each battery is a +12 volt
supply and has a useful life of up to 20 hours.
6. The apparatus of claims 1, 2 or 3, wherein the converter circuit
further comprises a circuit for monitoring the level of power
supplied and indicating when it falls below a threshold level.
7. The apparatus of claim 1 wherein the common power supply is a
rechargeable battery.
8. The apparatus of claim 1 wherein the voltage probe of one or
more of the plurality of voltage probe transmission links further
comprises first and second voltage probes for contacting the device
under test at first and second locations corresponding to the one
test point, wherein the first sensed voltage signal is the voltage
sensed between the first and second locations.
9. The apparatus of claim 8 further comprising a plurality of
dedicated power supplies such that there is one dedicated power
supply for each converter circuit.
10. The apparatus of claim 9 wherein each power supply is a
rechargeable battery.
11. The apparatus of claim 1 wherein each link further
comprises:
a receiver for receiving an optical signal and converting it to a
display signal corresponding to the first sensed voltage; and
an optical transmission line for transmitting the optical signal
from the converter to the receiver; and
further comprising a display for viewing the sensed signals at one
or more of the test points of the device under test.
12. The apparatus of claim 1 further comprising a base having a
plurality of positions for removably mounting the plurality of
converter circuits.
13. The apparatus of claim 12 wherein the base further comprises a
receptacle for the common power supply and plurality of conductors
for connecting the power supply to one or more of the plurality of
converters.
14. The apparatus of claim 12 wherein the base is radiation
hardened.
15. The apparatus of claim 12 wherein the common power supply is
associated with the base and further comprising a plurality of
shielded conductors for connecting the power supply to one or more
of the plurality of converters.
16. The apparatus of claim 12 wherein the common power supply
further comprises one or more rechargeable batteries associated
with the base, and a corresponding one or more plurality of
conductors for connecting the one or more batteries to the
plurality of converter circuits.
17. The apparatus of claim 16, further comprising a plurality of
power supplies such that each converter circuit also has an
internal power supply.
18. The apparatus of claim 17 wherein each converter circuit
further comprises a switch for switching out its internal power
supply when the converter circuit is connected to the common power
supply.
19. The apparatus of claim 17 wherein each of the internal power
supplies is a rechargeable battery having the same nominal output
voltage as the common power supply.
20. The apparatus of claim 1 wherein the common power supply is at
least one rechargeable battery.
21. The apparatus of claim 1 wherein at least one of the plurality
of voltage probe transmission links is an analog channel wherein
the converter circuit converts the sensed voltage signal at the
test point into an analog optical signal corresponding to the
sensed voltage.
22. The apparatus of claim 21, wherein analog channel has a
bandwidth from DC to 5 MHz.
23. The apparatus of claim 1, 2, 7, 8, 11, 12, 20 or 21 wherein the
voltage probe further comprises a circuit grabber for contacting
the one test point and an electrically overdamped input conductor
having a distributed impedance for conducting the voltage signal at
the circuit grabber to the converter circuit.
24. The apparatus of claim 23 wherein each input conductor is one
or more strands of a slurry of carbon and fluorocarbon materials
having a diameter on the order of 0.76 mm.
25. A method for monitoring the effect of a radiation field at a
plurality of test points of a device under test comprising:
providing a plurality of voltage probe transmission links, each
link including an electromagnetically transparent voltage probe for
contacting and sensing a first voltage signal at one test point and
a converter circuit for converting the first sensed voltage signal
to a corresponding optical signal;
providing power to the plurality of converter circuits from a
common power supply using a plurality of cables connecting said
plurality of converter circuits to the common power supply;
radiation hardening said common power supply and said pluralities
of cables and converter circuits;
contacting said plurality of test points with the plurality of
voltage probes so that the voltage probes do not inject signals in
any of the test points of the device; and
monitoring the voltage signals sensed at the plurality of test
points.
26. The method of claim 25 wherein providing power further
comprises providing one or more of the plurality of converter
circuits with a dedicated power supply.
27. The method of claim 26 further comprising enclosing each said
one or more converter circuits and its associated dedicated power
supply in a radiation hardened enclosure.
28. The method of claims 26 or 27, further comprising monitoring
the level of power supplied to each converter circuit and
indicating when the level of power falls below a threshold
level.
29. The method of claim 26 further comprises switching out the
internal power supply of a converter circuit when the converter
circuit is connected to the common power supply.
30. The method of claim 25 wherein the voltage probe of one or more
of the plurality of voltage probe transmission links is a first and
second voltage probes and contacting the device under test at a
test point further comprises contacting the first and second
voltage probes to first and second locations corresponding to the
one test point, and monitoring the voltage sensed between the first
and second locations.
31. The method of claim 30 further comprising providing a plurality
of dedicated power supplies such that there is one dedicated power
supply for each converter circuit.
32. The method of claim 25 wherein monitoring the plurality of test
points further comprise:
transmitting the plurality of optical signals from the converter
circuits out of the test radiation field;
converting the transmitted optical signals to display signals
corresponding to the plurality of first sensed voltages; and
displaying the monitored signals at one or more of the test points
of the device under test.
33. The method of claim 32 wherein at least one of the plurality of
voltage probe transmission links is an analog channel, wherein the
step of monitoring the voltage signals using said analog channels
further comprises converting the sensed voltage signal at the test
point into an analog optical signal corresponding to the sensed
voltage prior to converting the sensed first signal into an optical
signal.
34. The method of claim 25 further comprising enclosing each
converter circuit in an enclosure and mounting the plurality of
enclosures in a base having a plurality of positions for removably
mounting the plurality of enclosures.
35. The apparatus of claim 34 wherein mounting each enclosure to
the base further comprises connecting the power supply to the
converter circuit of each said mounted enclosure.
36. The apparatus of claim 34 wherein the common power supply is
associated with the base and is connected to one or more of the
plurality of converters by using shielded conductors.
Description
FIELD OF THE INVENTION
This inventions relates to methods and apparatus for testing the
susceptibility of devices, such as circuitry, to electromagnetic
interference (EMI).
BACKGROUND OF THE INVENTION
Analog and digital electronic circuitry and attendant wiring may
encounter serious operating difficulty in the presence of strong
electromagnetic radiation fields. Such radiation fields are
generally referred to as Electromagnetic Interference (EMI) fields.
The circuits and attendant wiring may be shielded and filtered to
provide some immunity to large EMI fields. Methods and apparatus,
therefore, are required to test the susceptibility of the circuits
and attendant wiring to EMI fields.
EMI testing is typically performed in shielded enclosures known as
"screen rooms" or faraday cages, which provide an electromagnetic
environment wherein only controlled EMI fields are present.
Controlled EMI fields include, but are not limited to radiated near
and far fields, stripline and TEM testing in the range of DC (more
typically 10 KHz) to 18 GHz.
Apparatus typically used inside the screen room includes current
probes attached to a harness wire and a coaxial cable which sends
the signals detected by the probes to a receiver outside the screen
room, where the effects of the EMI fields on the circuit are
determined. Current probes suitable for monitoring current during
EMI tests are commercially available. The Ailtech model number
91197-11 is one such device. Current probes, however, are not able
to measure signals in the device under test in many circumstances,
for example, at trace conductors of integrated circuits or into
open circuits. For such signals, voltage probes are better
suited.
To ensure the integrity of the screen room and the results of the
EMI tests, any voltage measuring apparatus within the screen room
should minimally perturb the controlled EMI fields and should be
energized by a signal from the device under test only. For example,
any test apparatus which might reradiate EMI fields impinging on
the device under test or might otherwise inject any noise into the
device under test must be avoided.
U.S. Pat. 4,939,446, which is assigned to the assignee of this
invention, refers to one such voltage probe transmission link that
is transparent to electromagnetic radiation fields for use in
screen room testing. The transmission link uses a voltage probe,
which includes a circuit grabber, such as a short insulated
conducting clip, which is connected to an electrically overdamped
input conductor. The circuit grabber is connected to the test point
of the device under test. The insulation on the clip surface is
coated, with any bright, metallic reflecting material, such as a
silver paint or foil, to shield the clip from impinging EMI fields,
thereby preventing the injection of signals into the device under
test by the clip. The other end of the input conductor is connected
to a hybrid electrical/optical data transmitter having a high
impedance input port, which also is located inside the screen room.
The transmitter has an optical output port that is connected to a
receiver by way of an EMI immune optical fiber. The receiver is
located outside the screen room where the effects of controlled EMI
fields on the device under test are monitored, outside of the test
electromagnetic radiation field.
The voltage probe input conductors may comprise a non-metallic
thread core that is impregnated with fine conducting particles and
a rigid, non-metallic insulating sheath. The electrically
overdamped input conductors have a high distributed resistance so
that they will not ring or tune at the frequencies of interest and,
therefore, will not pick up energy from the EMI fields. As a
result, the voltage probe transmission link may be used to monitor
voltages of a device under test in the presence of a strong EMI
field without effecting the device under test or the test results.
The disclosure of U.S. Pat. No. 4,939,446 is hereby incorporated in
its entirety herein by reference.
A commercial product, known by the tradename ETVL
(Electromagnetically Transparent Voltage Monitor Link System),
available from the assignee of this invention, Electronic
Development Inc., is a commercial version of the voltage probe
transmission link described in U.S. Pat. No. 4,934,446. The ETVL
product has a hybrid electrical/optical data transmitter that has a
single transmission channel that may have one of three signal
formats, namely analog, digital, and pulse stretched. Only one
signal format can be used at a time on the one transmission
channel.
The commercial ETVL device and the device described in U.S. Pat.
No. 4,939,446 monitor only a single test point and one voltage
waveform (single ended or double ended) of the device under test.
They also use one voltage probe for providing a return current path
from the transmitter to the device under test.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide
improved voltage probe transmission link apparatus and methods for
monitoring the effects of an EMI field on a device under test.
It is another object of the invention to provide a multi-channel,
electromagnetically transparent, voltage probe transmission link
system that can monitor simultaneously a plurality of voltage
signal waveforms of a device or system under test. It is another
object to provide for monitoring simultaneously the effect of
radiation fields at a plurality of locations without affecting the
device under test or the test results. It is another object to
monitor a plurality of waveforms at different locations along a
circuit path. It is another object to monitor a plurality of
voltage waveforms of a device under test inside a screen room to
determine the effect of a controlled EMI field.
It is a further object of the present invention to provide a
multi-channel voltage probe transmission link system that has a
rechargeable power supply. It is another object to provide a
plurality of voltage probe channels with a common (or shared) power
supply to provide for extended operation inside a screen room. It
is another object to provide each transmitter of a channel with a
dedicated power supply.
It is another object of the invention to provide a radiation
hardened, low power electrical to optical data transmitter for use
in a screen room during extended periods of time.
It is another object of the present invention to provide a variable
attenuator for attenuating the sensed voltage signals of the device
that are larger than the effective dynamic range of the electrical
to optical data transmitter. It is yet another object to provide a
voltage divider attenuator that is transparent to electromagnetic
radiation.
In accordance with this invention, a multi-channel
electromagnetically transparent voltage probe transmission link
system for sensing a plurality of voltage signals at a plurality of
test points of a device under test subjected to a radiation field
is provided. One aspect of the invention is directed to a
multi-channel system comprising a plurality of voltage probe
transmission link channels, each channel including two voltage
probes, each voltage probe comprising a circuit grabber and an
electrically overdamped input conductor in electrical contact with
the circuit grabber. The circuit grabber (or grabbers) contact the
device under test to monitor the voltage signals at one test point
and the input conductor (or conductors) electrically transmits the
sensed voltage signals.
Each channel also includes an electrical to optical transmitter for
converting the voltage signals transmitted by the input conductor
(or conductors) to an optical signal, and transmitting the optical
signal over a suitable transmission line. The optical transmission
line passes the optical signal out of the radiation field to a
receiver for receiving the optical signal. The receiver processes
the optical signal and provides a display signal corresponding to
the sensed voltage signal at the test point. The display signal is
then displayed and the waveforms may be evaluated to determine the
effect, if any, of the radiation field on the device under test at
the one test point.
Each voltage probe transmission link channel is associated with one
of the plurality of test points of the device under test. Because
the multi-channel link system is used to identify changes in the
monitored voltage signal waveforms caused by the radiation field,
it is not necessary that the displayed signals exactly display the
sensed voltages. Rather, the display signals need only reflect
relative changes in the monitored voltage signal waveforms as a
result of the radiation field used during the test.
Each transmitter may be provided with a dedicated power supply such
as a rechargeable battery. Also, the plurality of transmitters may
be connected to a common (or shared) power supply by relatively
short shielded conductors. Further, both a dedicated power supply
and a common power supply may be used. The common power supply may
be one or more discrete power supplies such that not all
transmitters are connected to the same power supply. If each
transmitter also has a dedicated power supply, the dedicated power
supply may be switched out, automatically or by a switch, when the
transmitter is connected to a common power supply. Preferably, the
dedicated power supplies are located internal to the
transmitters.
The plurality of transmitters, including any internal or common
power supplies, are radiation hardened, either as an integrated
system in an enclosure or as interconnected components, for the
test radiation field intensities (V/m) and frequencies of
interest.
Each transmitter is preferably releasably mountable on a common
base or frame. The term releasably mountable means that each
transmitter can be secured to and removed from a receptacle in the
base and used to monitor a voltage in both conditions. The
transmitter can be secured in place by any means, e.g., pins,
latches, keys, friction, bolts and nuts, etc. Securing each of the
transmitters to a common base provides for easy portability of the
equipment, for example, into, out of, and within a screen room. It
also provides for radiation hardening the base with the plurality
of transmitters secured to the base.
In embodiments where a common power supply is used, the common
power supply may be built into the base, and the base, the common
power supply, and the plurality of cables connecting the common
power supply to the transmitters may be radiation hardened as an
integrated assembled unit.
That each transmitter may be removed from the base provides for
locating each transmitter proximate to the test point it is to
monitor. This is advantageous where the device under test is a
large object, such as an automotive vehicle or its electrical
system or a local area network of computers, and the distance
between two transmitters for two test points being monitored is
greater than the desired length for the input conductor of the
voltage probe. This in turn provides for using the same length
input conductor for each voltage probe, and maintaining that length
to less than a meter. This is advantageous in view of distributed
resistance of the input conductor material, which results in a
resistance that is directly proportional to its length. As noted,
each transmitter also may be advantageously provided with an
internal power supply. Thus, if the cable connecting the
transmitter to the base power supply becomes problematic with
respect to radiation hardening, the cable may be omitted and the
internal power supply switched in.
Removability of the converters also provides for using less than
the full plurality of transmission channels, rapid replacement of a
transmitter that is in need of service (or a recharge when
operating on an internal battery) and quickly and easily changing
the mixture of channel types of the plurality of channels, as
between analog and digital transmitter channels. This is
particularly advantageous when complicated digital integrated
circuits having analog sensors is being tested and an EMI
susceptibility problem has been identified, yet needs to be better
isolated along a signal path having analog and digital signals.
An advantage of a common power supply is that it may be larger and
have a longer useful life than using a plurality of dedicated power
supplies which preferably are small enough to fit into the
transmitter enclosure. For example, the common power supply may be
a heavier 4.8 amp-hour rechargeable battery having a battery life
of 30 hours when connected to two transmitters. In contrast,
suitable internal power supplies for each transmitter may be
lighter, smaller, and have a shorter useful life, for example, a 12
volt 0.6 amp-hour battery having a useful life of seven hours.
Further, the internal battery may be automatically switched in if
the common power supply becomes discharged below a threshold
voltage or disconnected, thereby extending the useful life of the
voltage probe transmission link channels. Similarly, if any
transmitter is not being used, it may be automatically powered down
or manually switched off so that it does not unnecessarily drain
the power supply.
Also, the common power supply (with or without the internal
battery) may be located adjacent the base, or outside of the
radiation field and coupled to the transmitters using appropriately
shielded cables. In yet another embodiment, the common power supply
may be derived from conventional line current that is converted to
the DC voltage level used by each system, preferably a regulated DC
voltage.
Preferably, each of the transmitters is provided with a plug or a
receptacle that is compatible with a corresponding receptacle or
plug in the base so that when the transmitter is secured to the
base, it is connectable to a common power supply. The connection
may be automatic through the plug/receptacle connection, or it may
be controlled by a suitable switch or conventional shielded cables
and connectors.
Each of the plurality of voltage probe transmission links also
comprises a receiver, which is located out of the effective range
of the test radiation field. Each receiver receives the optical
signals corresponding to the sensed voltage of one test point from
the optical transmission line, and processes the optical signals to
produce a display signal. The plurality of receivers are preferably
releasably mountable on a common base and are respectively
connected to a suitable device (or devices) for displaying the
plurality of display signals corresponding to the plurality of
voltage signals monitored at the different test points of the
device under test. A suitable display may be a multi-channel
display device or a plurality of single channel display devices,
for example, one or more single or multi-channel oscilloscopes,
spectrum analyzers, voltage meters, or similar devices.
Surprisingly, it was discovered that efforts to multiplex the
optical signals corresponding to the plurality of test points, to
permit use of a single optical fiber passing out of the radiation
field, tended to mask susceptible device voltage signal waveform
changes that occurred during RF testing. In particular, a
prohibitively high sampling rate would be required to detect small
waveform changes in a waveform having a 30 MHz frequency. Although
multiplexing may be useful for a two or three channel transmission
link system at low radiation field frequencies, such a system is
not practical or sufficient to satisfy the commercial needs of the
users who require, for example, six (or more) channels to monitor a
device under test at frequencies up to 18 GHz.
Another aspect of the present invention is directed towards an
attenuator for attenuating a sensed voltage signal to a range
suitable for processing by a device having a limited input range,
over a frequency range of interest. One embodiment of this aspect
of the invention concerns an attenuator for attenuating the sensed
voltage at the device under test for processing by a low power
transmitter, which has a limited input signal range. One such
attenuator includes a voltage probe and a length of an electrically
overdamped conductive wire having a distributed impedance
(resistance and capacitance) along its length, the length
connecting the input conductor of the voltage probe to a ground
(virtual or actual). This results in the voltage input at the probe
being divided across the first length and the distance between the
voltage source and the location where the first length is connected
to the voltage probe input conductor. Thus, by adjusting the
relative location of the connection along the probe input
conductor, or by adjusting the length of the first length (or
both), the magnitude of the voltage source may be attenuated by a
selectable amount. This provides for a signal, corresponding to the
monitored voltage signal, that has a relatively full scale peak to
peak swing with respect to the transmitter input capacity.
Preferably, the power supply (dedicated and/or common) is monitored
by a battery charge monitor to provide an indication of the net
charge on the power supply. This is important because if the power
supply voltage falls below a preselected level, e.g., 10 volts, the
transmitter circuits may not operate in a linear manner, and, if
undetected, could transmit distorted signals that could be mistaken
for signals affected by a radiation field. The battery charge
monitor could be used to trigger a switch to change automatically
between a common power source and an internal power source such
that the internal power source is used as a backup power
supply.
Another aspect of the present invention concerns another
improvement to U.S. Pat. No. 4,939,446 concerning the electrical to
optical transmitter and monitoring the voltage waveform at a test
point of the device under test. In this aspect, the input circuits
to the differential amplifier of the transmitter are provided with
a centertapped interconnection. This centertap provides a return
current path for the signals of the device under test that are
sensed by a voltage probe. Accordingly, the need for a reference
ground return voltage probe connecting the device under test to the
transmitter centertap voltage has been eliminated. Thus, no more
than two voltage probes are now needed to monitor a differential
output voltage signal, in place of the three probes previously
required. For monitoring single ended output voltage signals, the
second input circuit of the transmitter is preferably connected to
the ground of the device under test.
In a preferred embodiment, the centertap voltage return is obtained
by respectively inputting the sensed voltage signals from the two
input conductors into two potentiometers at the input circuits of
the transmitter, such that the other ends of the potentiometers are
connected to a common centertap voltage. Advantageously, this
centertap return simplifies connection of each channel transmitter
to a test point of devices under test and reduces the number of
voltage probes required. This reduces the cost of the multi-channel
device and the time required to select and connect the voltage
probes to the test point of the device under test or reconnecting
the voltage probes from one test point to another. The savings are
multiplied by the number of channels used.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features of the invention, its nature and various
advantages will be apparent from the accompanying drawings and the
following detailed description of the invention in which like
reference numerals refer to like elements and in which:
FIG. 1 is an isometric view of a test set-up for a multi-channel
EMI transparent voltage waveform monitor link in accordance with a
preferred embodiment of the present invention;
FIG. 2 is a block diagram of an alternate embodiment of the test
set-up of FIG. 1;
FIG. 3 is a circuit block diagram for an analog channel voltage
probe transmission link in accordance with an embodiment of the
invention;
FIG. 3A is a circuit schematic for the analog channel electrical to
optical transmitter of FIG. 3;
FIG. 3B is a circuit schematic for the analog channel optical to
electrical receiver of FIG. 3;
FIG. 4 is the battery charge monitor circuit of FIG. 3A;
FIGS. 5A, 5B, 5C, and 5D are representations showing the effects of
a radiation field for two test points of a device under test before
and after the effects appear;
FIG. 6 is a representation showing the effects of the radiation
field for the two test points of the device under test of FIG. 5
after electromagnetic susceptibility is corrected; and
FIG. 7 is a diagram of a voltage divider transparent to
electromagnetic radiation in accordance with a preferred embodiment
of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. shows a preferred embodiment of the multi-channel transmission
link system of the present invention. Such a system includes a
plurality of channels, e.g., six or more, of which only four
channels are shown. The four channels are respectively designated
by the suffix letters a, b, c, and d. These suffixes are used
throughout the specification to designate corresponding elements of
the same channel. As it will appear from the context of the
discussion, the suffixes may be omitted when a characteristic
common to each of the channels is discussed.
The system shown in FIG. 1 has four transmitter modules 10a, 10b,
10c and 10d. Transmitters 10b, 10c, and 10d are shown mounted to a
common base 20. Base 20 is illustrated in FIG. 1 as an L shaped
rack, but may have any other convenient shape that is compatible
with retaining transmitters 10. Transmitter 10a is shown removed
from base 20. Receptacles 21a and 23a on base 20 are visible.
Receptacle 21 may be used for alignment and correct seating of
transmitter 10. Receptacle 23 may be used when coupling transmitter
10 to a common power supply (e.g., supply 80 in FIG. 2).
Each transmitter 10 is used to monitor a different test point,
respectively designated by reference characters a, b, c, and d, of
a device under test 100. Referring also to FIG. 3, each transmitter
10 has a pair of voltage probes 301 and 302 such that a pair of
circuit grabbers 11 and 12 are respectively connected across the
double ended output of the test point in device 100 for monitoring
the voltage at the test point. Each of the circuit grabbers 11 and
12 are respectively connected to transmitter 10 by electrically
overdamped conductors 14 and 15. If the test point is a single
ended output, voltage probe 302 need not be used.
Regarding the circuit grabbers, they are metallic grabbers inserted
at the end of the overdamped conductors. The grabbers are coated
with a paint that reflects E-fields up to 200 V/m over sweep
frequencies as high as 18 GHz. A hard baked top coat of plastic,
e.g., poly-urethane, such as Sherwin-Williams brand Polane B, is
applied over the reflective paint to prevent scratches that might
destroy the reflecting capacity of the paint. Untreated grabbers
become more sensitive to pickup at frequencies above 1.0 GHz.
To connect to an exposed wire or circuit lead, a clip type grabber
may be used. To connect to a harness cable coated with insulation,
a pin type grabber that penetrates the insulation may be used.
Thus, according to the present invention, a plurality of channels
having respective voltage probe pins may be inserted at different
lengths along a harness to check for resonances. Other circuit
grabber shapes and configurations may be used for securely
fastening to the test point of the device. This includes wires
having a reflective coating soldered to test points. The latter is
particularly useful if the device under test is being moved during
the test. Each circuit grabber may be one of the clip or pin as
described in U.S. Pat. No. 4,939,446.
Regarding the electrically overdamped input conductor, it is
preferably a non-metallic material made from a glass and carbon
slurry that is wrapped (or coextruded) with a rugged nylon
protective sheath and conducts the monitored waveform therethrough
by the well known displacement current mechanism. These conductors
are transparent to electrical fields as high as 200 V/m in a
frequency range between 10 KHz and 18 GHz, and to magnetic fields
between 30 Hz and 80 KHz. For example, an acceptable probe input
conductor is known by the tradename FLUOROSINT.RTM. 719, available
from the Polymer Corporation of Reading Pa. It has a
carbon/fluorocarbon core that is 0.030 inches (0.76 mm.) in
diameter and enclosed in a transparent nylon insulating cover to
yield an outer diameter of 0.040 inches (1.02 mm.), and a
resistance per unit length of 20,000 to 30,000 ohms per inch (7874
.OMEGA./cm. to 11811 .OMEGA./cm). The input conductor material has
a small amount of distributed capacitance that causes sensed
voltage signal waveforms, monitored from device 100, to roll off at
high frequencies. This roll-off is compensated for by using wide
bandwidth amplifiers from DC to 40 MHz in the transmitter circuits.
The input conductor may be a single strand or multiple strands,
e.g., 4 to 5 strands, connected in parallel to lower the effective
impedance, to increase its structural rigidity as a cable. A
preferred overdamped input conductor and circuit grabbers, all of
which are transparent to the radiation field, are further described
in U.S. Pat. No. 4,939,446.
Referring to FIGS. 1-3, each transmitter 10 includes an electrical
to optical converter circuit 30. Converter circuit 30 has inputs 31
and 32 for receiving the voltage drop across circuit grabbers 11
and 12, as transmitted by conductors 14 and 15, and power from a
power supply, and has as an output an optical signal.
The optical signal is coupled to an optical transmission line 50,
e.g., a conventional fiber optic cable (or cable bundle), that is
transparent to the radiation field at the frequencies of testing.
Transmission line 50 has a length that is sufficient to extend from
transmitter 10 out of the radiation field to which the device 100
is exposed, for example, 10 meters or more. The power supply may be
an internal battery 40 (see FIG. 1), a common battery 80 that is
shared by one or more other transmitters 10 (see FIG. 2) or both
(not shown).
In the embodiments shown in FIGS. 1 and 2, device under test 100
includes a printed circuit board 110 having a microprocessor device
120 and a receiver logic circuit 130 that are connected by a lead
140 and a separate power supply 150 that is connected to circuit
130 by a harness 160. Thus, test points a, b, c and d represent
different locations along one or more circuit paths of device
100.
The device 100, the transmitters 10a, 10b, 10c, and 10d, and base
20 are physically located inside a conventional screen room 200.
The screen room 200 has a plurality of antenna devices 210 (four
such antennas 210 are shown) for providing a controlled EMI field
(i.e., a controlled radiation field) for testing device 100 for the
effects of the EMI field, i.e., susceptibility to electromagnetic
radiation, electromagnetic emission, or both. It is to be
understood that the invention is applicable to testing the effect
of any radiation field on any device (or system) under test,
whether or not the test is performed in a screen room, and whether
or not the test is conducted using a controlled radiation field, a
near field emitted from a waveguide, a strip line, a TEM field, or
an ambient (near or far) radiation field.
Referring to FIG. 1, the plurality of optical transmission lines,
e.g., lines 50a, 50b, 50c, and 50d, are passed through a
waveguide-beyond-cutoff filter 220 located in the enclosure of the
screen room 200, to a like plurality of receivers 60, namely
receivers 60a, 60b, 60c and 60d. Filter 220 is used to prevent
transmission of electromagnetic radiation in either direction
through the aperture, which transmission may adversely affect the
test.
Each of the receivers 60 is used to convert the optically
transmitted signal to a display signal, which can then be
transmitted over a compatible conductor 68 for display on a display
70. Preferably, the display signal is an electrical signal and
conductor 68 is a conventional coaxial cable, such as RG-58/U type
cable. Receivers 60 are preferably removably mounted in a rack 65.
Receivers 60 also may be provided with dedicated 12 volt DC power
supplies or a common 12 volt power supply. These power supplies may
be rechargeable batteries internal to each receiver or common to
the receivers, or derived from one or more AC to DC power supply
devices operating from a wall outlet. Preferably, the receivers 60
are individually switched so that the receivers 60 for unused
channels may be turned off. Removability of the transmitters 10 and
receivers 60 also allows for replacing a unit in need of service or
in need of a battery charge with a fully operational and charged
unit, or changing a channel transmission link from a digital to an
analog channel or vice versa, without significantly delaying or
interfering with testing of the device under test.
Each channel of the multi-channel system may be an analog channel
or a digital channel. Such types of channels are known and have
been used in the aforementioned commercial ETVL product, but only
in a configuration that has both channel types such that the user
may select and use only one channel type at a time. Thus, in the
present invention, each transmitter 10 is a single type of channel
and both the number of channels and the type of channel may be
selected to permit optimum monitoring of a wide variety of voltage
signal waveforms simultaneously from device 100.
Analog channels are particularly useful for transmitting monitored
voltage waveforms at frequencies at and below about 5 MHz. Digital
channels are particularly useful for transmitting frequencies above
5 MHz, in which a fast rise and fall time are used, e.g., 25
nanoseconds or less. The digital channel provides a pulse
stretching function, taking advantage of the distributed
capacitance of the voltage probe input conductor. The pulse
stretching ratio is about a 4:1 ratio. This is particularly useful
for viewing waveforms comprising narrow pulse periods that are long
with respect to pulse width.
As shown in FIG. 1, display 70 may be a multi-channel scope capable
of displaying the monitored signals from test points a, b, c, and
d, simultaneously. Display 70 is preferably capable of monitoring
the waveforms of device 100 before and after the application of the
radiation field of the test or, more specifically, the radiation
field at two intensity levels. Typically, one radiation field
intensity level will be selected because no effects of the
radiation field appear in the monitored voltage signal waveforms
and a second intensity level will be selected because the affect of
the radiation field does appear in some, if not all, of the
monitored test point signals.
Voltage waveform changes provide the user with both test failure
and diagnostic information during signal tracing. This information
can be used to isolate the electromagnetic coupling or emission
problem in the device 100, i.e., isolate the problem to a specific
circuit component, element, or lead. Indeed, the present invention
provides for simultaneously monitoring different points along a
signal path in a circuit or device and identifying the likely
source of the EMI based on the relative changes in the plurality of
voltage signal waveforms along the signal path. Thus, the likely
problem can be more quickly located. Further testing to isolate
more specifically the problem can be conducted. Thereafter,
suitable corrective steps can be implemented, for example,
insertion of a suitable EMI filter to resolve the EMI
susceptibility or emission problem in a given application of device
100.
Referring to the embodiment of FIG. 1, each transmitter 10 has a
dedicated power supply 40, which is preferably a 12 volt
rechargeable lead acid battery. Preferably, the dedicated power
supply 40 is located internal to the enclosure of transmitter 10,
which also encloses converter circuit 30 and the enclosure is
radiation hardened. In such an embodiment, the battery harness
connecting battery 40 to the power input reference 325 of converter
circuit 30 is located inside the hardened enclosure and need not be
separately shielded or electromagnetically transparent.
FIG. 2 shows an alternate embodiment of a portion of the test
set-up of FIG. 1 wherein each transmitter 10' receives power from a
common power supply 80 over one of a plurality of conductors 82. In
this embodiment, transmitter 10a', which is removed from base 20,
is provided with power from power supply 80 by an EMI shielded
cable 82a. It is desirable to keep cable 82a as short as
possible.
Preferably, the common power supply 80 is associated with the base
20 such that it is connected to the base 20 or enclosed interior to
base 20. This provides for enhanced radiation hardening of the
base, common power supply, and the plurality of cables as an
integrated unit. For those transmitters 10' that are removed from
base 20 by a distance greater than 15 meters, it may be desireable
to provide such transmitters 10' with a dedicated, more preferably,
internal, battery 40. This avoids the risks of a long cable 82
coupling to the radiation field or emitting radiation and affecting
the test results. In such circumstances, use of an internal battery
will optimize radiation transparency.
A larger common power supply (not shown), located separate from the
base 20, or even outside of the radiation field also may be used
with appropriate shielded cables. Preferably, the common power
supply is one or more rechargeable batteries located in the
radiation field and radiation hardened. This is to avoid the
problems inherent in using an AC to DC power supply in an
electromagnetic, particularly an RF, field and using long battery
harnesses.
Importantly, the multi-channel voltage probe transmission link
system, and in particular the transmitters 10 (or 10') and their
power supply, namely either a dedicated internal battery 40, a
common battery 80, or a remote battery or power supply (not shown
in FIGS. 1 and 2), is to be radiation hardened to at least the
radiation intensity levels for the frequencies of the test. This is
so that the multi-channel system will not radiate EMI during the
test or couple to radiation from the radiation field, which could
perturb the radiation field or inject signals into the device under
test and adversely affect the quality of the test.
Typically, each of transmitters 10 must be hardened against both
near and far fields to 100 volts per meter (V/m) over the frequency
range of from 10 KHz to 18 GHz. Base 20, together with any internal
or external common power supply 80 and cables 82, is similarly
radiation hardened with the transmitters 10 secured to and removed
from base 20. It also is preferred that the test apparatus be
compatible with the conventional TEMPEST and EMP requirements.
Referring to FIGS. 3 and 3A, analog channel converter circuit 30 is
a low-gain, direct-coupled, optical transmitter amplifier circuit
which transmits analog waveforms through the transmitter output LED
310, into optical cable 50, and to a compatible analog receiver
circuit 60. LED 310 is operated along a linear portion of its
output characteristic so that the intensity of the light emitted is
directly related to the magnitude of the voltage drop at the test
point.
Analog converter circuit 30 is connected to a test point of device
100 (not shown in FIGS. 3, 3A) using two voltage probes 301 and 302
(not shown in FIG. 3A). Voltage probe 301 includes circuit grabber
11 and a first length of overdamped input conductor 15 and voltage
probe 302 includes circuit grabber 12 and a first length of
overdamped input conductor 14. Preferably, the first length of
input conductor 14 and the first length of input conductor 15 are
the same. Regarding the test point signal of device 100, grabber 12
is connected to one of the "test point" and a "return path" in
device 100, which may be a circuit ground of device 100, and
grabber 11 is connected to the other of the test point and the
return path, thereby to obtain a differential signal waveform which
is provided to inputs 31 and 32 of converter circuit 30. The
differential waveform is input to a differential amplifier circuit
320.
Referring to FIG. 3A and amplifier circuit 320, the signal input at
input 31 is passed across an input resistor 311, into one end of a
potentiometer Pi, and out the wiper contact of potentiometer P1
into the noninverting input of amplifier A1. The input signal at
input 32 is similarly passed across an input resistor 312, a
potentiometer P2, and the noninverting input of an amplifier A2.
Input resistors 311 and 312 are each preferably 2.1 K.OMEGA. and
are optionally used to prevent bad overdriving of converter circuit
30 in the event that potentiometers P1 and P2 are not properly
adjusted. Potentiometers P1 and P2 are ganged together to provide
the same resistance. The ends of potentiometers P1 and P2 on the
other side of their wiper contacts from the input signals are
connected to a common centertap CT, and thereby provide a return
current path to the test point of device 100. Thus, the problem of
requiring a separate voltage probe to provide such a return current
path to the device under test, as disclosed in U.S. Pat. No.
4,939,446, is overcome.
Potentiometers P1 and P2 each may be a 50 K.OMEGA. potentiometer,
although it is believed that other potentiometers having resistance
values up to 1.0 M.OMEGA. may be used. In operation, potentiometers
P1 and P2 are first placed in their full resistance positions,
ganged together, and then adjusted to reduce the resistance to a
level that provides the desired waveform amplitude range into
amplifiers A1 and A2.
Amplifiers A1 and A2 are preferably provided with the same voltage
to current converter amplifier configuration, i.e., a unity gain
buffer amplifier having the output pin 6 fed back to inverting
input pin 2. A +12 volt supply is provided at pin 7 of amplifiers
A1 and A2 from the +12 volt battery, e.g., battery 40 (FIG. 1) or
80 (FIG. 2), obtained as illustrated from pin 3 of a commercial EMI
filter 340 (and described below). A virtual ground return is
provided at pin 4 of amplifiers A1 and A2, obtained from node VG,
corresponding to the return path to the +12 volt supply and thus
the point of lowest potential in circuit 30. In this embodiment,
amplifiers A1 and A2 are insensitive to any relative voltage drift
in the power supply.
The outputs of amplifiers A1 and A2 are respectively fed to the
inverting input at pin 2 and the noninverting input at pin 3 of a
differential amplifier A3. Amplifier A1 output at pin 6 is passed
across a 1.8 K.OMEGA. resistor 321 into pin 2 of amplifier A3.
Amplifier A2 output at pin 6 is passed across a voltage divider
circuit of potentiometer P3 and resistor 322 connected to the
centertap CT (described below) into pin 3 of amplifier A3.
Potentiometer P3 is a 5 K.OMEGA. potentiometer and resistor 322 is
a 1.8 K.OMEGA. resistor. They are used to provide a common mode
rejection for the selected input resistance to amplifiers A1 and
A2. In other words, with potentiometers PI and P2 at their selected
values and terminals 31 and 32 tied together, potentiometer P3 is
adjusted to obtain a balance between inputs 31 and 32. Amplifier A3
is provided with a resistor 323 of 1.8 K.OMEGA. in the feedback
loop to provide unity gain. Other resistance values could be used,
for example, to provide a low gain other than unity.
The output of differential amplifier A3 at pin 6 is then input to
the inverting input at pin 2 of amplifier A4. Amplifier A4 is
configured as a unity gain inverting amplifier, having a 1.8
K.OMEGA. resistor 324 at the inverting input and a 1.8 K.OMEGA.
resistor 326 in the inverting feedback loop. Amplifiers A3 and A4
are also provided with bias supplies of +12 volt at pin 7 and
virtual ground at pin 4. The noninverting input of amplifier A4 is
connected to the centertap CT.
The centertap CT is provided by a voltage regulator 345. Voltage
regulator 345, such as a MC7805, manufactured by Motorola, Inc.,
converts the +12 volt supply and provides a regulated +6 volts
output across a 0.1 .mu.f decoupling capacitor 346 which is
connected to virtual ground at node VG. Thus, the +6 volt centertap
CT is electrically connected to the noninput leads of
potentiometers P1 and P2, to resistor 322 and to the noninverting
input of amplifier A4.
The output of amplifier A4 is passed to the base of transistor Q1
which is a an RF transistor, such as model 2N3904 or the
equivalent. The collector of transistor Q1 is connected to the +12
volt supply across decoupling resistor 350 and capacitor 351 which
are respectively 47 .OMEGA. and 0.1 .mu.f. Capacitor 351 is tied to
the virtual ground at node VG. The emitter current of transistor Q1
is passed across a DC bias resistor 353 of 390 .OMEGA. and into
light source 310. Light source 310 is preferably a commercial light
emitting diode device, model No. HFBR 1404, manufactured by Hewlett
Packard, having a nominal wavelength of 820 nm.
EMI Filter 340 is inserted between converter circuit 30 and the two
leads connecting the circuit to the power supply. It is used to
suppress electromagnetic susceptibility radiation over the
frequency range of interest. It may be any filter suitable for such
purpose, and preferably is model BNX002, available from MURATA
Manufacturing Co., Ltd., Savannah, Ga., which has a flat filter
response of between 0.01 and 1.0 GHz.
In the design of converter circuit 30 of FIG. 3A, it is important
that each element connected to the virtual ground be directly
connected to node VG by a separate dedicated conductor (not shown),
i.e., to the output pin 4 of EMI filter 340. This will minimize the
circuit noise to a level that is at about 10 mV or less. The
connection between the +6 volt centertapped output CT of the +6
volt regulator 345, may, but need not, be made by a separate
conductor to each of potentiometers P1 and P2, resistor 322, and
amplifier A4 (not shown).
The use of amplifier A4 as an inverting amplifier in converter
circuit 30 provides for simplifying the design of the compatible
receiver 60.
Referring to FIGS. 3A and 4, the converter circuit 30 also includes
a battery charge monitor 365, connected between terminals 3 and 4
of EMI filter 340. Monitor 365 preferably comprises a bar graph
driver device 366, such as part no. LM3914N, manufactured by
National Semiconductor and three different colored light emitting
diodes (LEDs) 367, 368 and 369. Device 366 is configured to turn on
one of the LEDs when the power supply is within the range of one of
the LEDs. Thus, LED 367 is preferably a red LED, e.g., Dialco part
no. 558-0102-001, and is illuminated when the power supply is less
than 11 volts; LED 368 is preferably a yellow (amber or orange)
LED, e.g., Dialco part no. 558-0202-002, and is illuminated when
the power supply is between 11 and 12.8 volts; and LED 369 is
preferably a green LED, e.g., Dialco part no. 558-0302-001, and is
illuminated when the power supply is between 12.8 and 13.6
volts.
Monitor 365 is configured as shown in FIG. 4, with each of the LEDs
367, 368 and 369 respectively connected in series between the power
supply and 5.1 K.OMEGA. resistors and to pins 12, 11 and 10 of
device 366. Regarding device 366, pin 3 is connected to the power
supply; pins I, 18, 17, 16, 15, 14, 13 and 12 are tied together;
pins 2, 4 and 8 are tied to the virtual ground; pin 5 is connected
to a bias voltage circuit including a voltage divider connected
between the +12 volt power supply and virtual ground, comprising a
100 K.OMEGA. potentiometer P and 10 K.OMEGA. resistor, for setting
the voltage thresholds for turning on and off the different LEDS
367, 368 and 369; pins 6 and 7 are tied together; and pins 6 and 8
are tied together by a 1 K.OMEGA. resistor LEDs 367, 368 and 369
are preferably visible to the operator, more preferably
conveniently located on or visible through the enclosure of
transmitter 10. Circuit 366 also may be connected to a switch, for
example, to actuate an audible alarm or to power down automatically
the transmitter 10 when the voltage falls below a threshold level,
e.g., when the red LED is illuminated.
Converter circuit 30 thus has an automatic gain and bias adjust
circuit that maintains its output in the linear operating region of
the light source LED 310. Amplifiers A1 and A2 provide unity gain
between input terminals 31 and 32 and the input of amplifier A3,
and a low gain at the output of transistor Q1, which output drives
LED 310. It is capable of operating with a power supply of between
10 and 15 volts. Below 10 volts, there may be a loss of linearity
in the circuit that could lead to inaccurate signal conversion and
distorted waveforms. Accordingly, the threshold level of battery
charge monitor is set somewhat above the voltage where loss of
linearity may occur. The circuit is a low power circuit and
requires only about 70 mA during operation. Accordingly, one
converter circuit 30 can operate for about 15 hours on a 1.2 A-H
rated +12 volt battery. The resultant DC drift is thus maintained
in the mV range and the noise level is on the order of 10 mV.
Referring to FIG. 3, an embodiment of an attenuator that may be
used is shown. In this embodiment, a switch S1 is used to switch a
short length of material R1 (i.e., the same non-metallic,
electrically overdamped input conductor of conductor 15) to connect
input terminal 31 of amplifier A1 to the virtual ground. Although
shown interior to transmitter 10, the material R1 could be located
outside of transmitter 10.
The resistance of material R1, i.e., its length, is selected to be
a fraction of the length of conductor 15. Thus, when material R1 is
placed across the series resistance of probe 301, it reduces its
input voltage by the fraction. A preferred fraction is one tenth.
Thus, R1 is 1.9 inches for a voltage probe input conductor 15
length of 19 inches. When actuated, switch S1 allows higher than
normal TTL, CMOS and other common device 100 digital voltage
waveforms to be monitored.
Referring to FIG. 7, another attenuator design is shown. In this
embodiment, a short length of material 17 (i.e., the same
non-metallic, electrically overdamped conductor of conductor 15)
connected between a selected location A on conductor 15 and the
virtual ground VG. The proportions of conductor 17 and the relative
distance of point A from circuit grabber 15 are selected so that
the combination of conductor 15 and conductor 17 form a voltage
divider. Thus, by adjusting position A, the point of electrical
contact, the voltage divider value is selected and then the voltage
signal monitored at the test point can be attenuated to within the
desired limits of converter circuit 30, e.g., .+-.6 volts, more
preferably, .+-.3 volts. Such an attenuator also is
electromagnetically transparent and thus may be located internal or
external to the transmitter 10 enclosure.
Thus, the present invention provides for using an attenuator to
monitor signals from device 100 that are as high as 150 volts, peak
to peak, DC or AC, without driving amplifiers A1 and A2 into
saturation, and with minimum distortion, i.e., below 10 mV of
noise.
A compromise between low value resistors for maximum bandwidth and
minimum current drain from the battery resulted in an amplifier
circuit as illustrated in FIG. 3A that is capable of processing a
sensed 5 volt signal to produce a 1.0 volt output signal at 5 MHz,
and to produce a 0.5 volt output signal at 10 MHz has a maximum
gain bandwidth product of 1.0 MHz and a battery current drain on
the order of 80 milliamperes.
Referring now to FIGS. 3 and 3B, a receiver 60 for receiving the
optical signal transmitted from converter 30 of module 10 is shown.
A light detector 380 is used to convert the analog optical signal
to an analog voltage signal. Detector 380 is preferably an
integrated photodiode-amplifier circuit, such as part no. HFBR
2404, available from Hewlett Packard. As shown in FIG. 3B, detector
380 has a variable DC offset voltage, provided by circuit 390, that
is used to compensate for changes in the DC operating levels of the
direct coupled amplifiers of converter circuit 30. Circuit 390 has
a potentiometer P4 and a 0.1 .mu.f capacitor 391, connected in
parallel to a virtual ground at node VG2 (described below).
Detector 380 has a regulated +5 volt supply, which is provided by a
+5 volt regulator 395, and which is passed across decoupling
resistor 396 and capacitors 397 and 398. Resistor 396 is 47
.OMEGA., and capacitors 397 and 398 are each 0.1 .mu.f.
In the preferred embodiment, offset circuit 390 is used to overcome
a discovered limitation in design of the HP HFBR 2404 device, which
has an AC coupled output that limits its DC output to a maximum
voltage of +0.43 V. This voltage level is inadequate to drive RF
transistor Q2, which is a model 2N3904 or equivalent transistor, to
maintain a DC bias voltage which is desirable for monitoring the
effects of radiation fields.
As is noted below, one possible effect is an inversion of the
monitored voltage signal. Thus, if the DC bias level of receiver 60
were 0, the magnitude of the inversion could not be monitored or
evaluated. The same is true for the output of receiver circuit 30.
Circuit 390, however, provides for adjusting potentiometer P4 to
raise the DC potential of device HFBR 2404 to a level sufficient to
drive transistor Q2, e.g., between 0.5 and 1.5 volts. Potentiometer
P4 could be replaced with a fixed resistor when a desired bias is
obtained. For example, potentiometer P4 could be set at or replaced
with a resistor of 202 .OMEGA. to obtain output of detector 380 at
75 volts. This provides for a bias level of +3 volts output at the
collector of transistor Q3, which is adequate to display effects of
the radiation field.
The output of the HP HFBR 2402 device is then passed to the base of
transistor Q2. The collector of transistor Q2 is provided with a
regulated +6 volt supply from +6 volt regulator 392 that is passed
across a current limiting resistor 393 of 750 .OMEGA. and a
capacitor 394 (0.1 .mu.f). Capacitor 394 is connected to virtual
ground VG2. Each of regulators 392 and 395 are provided with a +12
volt supply at input 410 (across on/off switch 52 and fuse F) and a
virtual ground at node VG2. The virtual ground VG2 is at input 420
and is the return current path to the +12 volt DC power supply for
receiver 60. As is the case with conductor circuit 30, every
element of circuit that is connected to the virtual ground is shown
connected to node VG2 by a separate conductor to minimize
noise.
The emitter of transistor Q2 is connected to virtual ground VG2
across a bias level circuit including potentiometer P5 (1.0
K.OMEGA. potentiometer) in parallel with a capacitor 399 (0.01
.mu.f). Potentiometer P5 is adjusted to set the proper bias level
for transistor Q2 to have the quiescent operating point centered on
the load line of the transistor.
The collector of transistor Q2 is connected to the base of
transistor Q3, which is a 2N3904 transistor or equivalent. The
collector of transistor Q3 is connected to the +12 volt supply over
decoupling resistors 401, each of which is 47 .OMEGA., and
capacitors 402, each of which is 0.1 .mu.f, as illustrated in FIG.
3B. The emitter of transistor Q3 is connected to virtual ground at
node VG2 across resistor 403, a 330 .OMEGA. resistor, and to output
V.sub.OUT across a resistor 404, a 50 .OMEGA. resistor. Other
decoupling resistors 401 (47 .OMEGA.) and capacitors 402 (0.1
.mu.f) are illustrated in FIG. 3B.
The +6 volt regulator 392 and +5 volt regulator 395 are used to
provide drift control for circuit 60, to maintain DC drift to less
than 10 mV. The receiver circuit 60 provides output signals that
vary by about 3-4 volts in response to the optical input signal.
Thus, it is preferred to use a display device 70 that has an
adjustable gain, such as an oscilloscope or multi-channel
oscilloscope, to amplify the output signal at V.sub.OUT to a
desired amplitude peak to peak swing range, e.g., .+-.6 volts.
Circuit 60 also includes a circuit for adjusting the bias level of
the signal V.sub.OUT by incorporating a potentiometer P6 across the
input terminals 410 and 420 and using the potentiometer wiper as a
reference output REF. Thus, by adjusting the potentiometer P6, the
DC bias level of signal V.sub.OUT may be selected without affecting
the waveform of signal V.sub.OUT. Potentiometer P6 preferably is a
3 K.OMEGA. potentiometer.
Receiver circuit 60 is preferably provided with a 50 ohm output
which may be varied as needed to be coupled to a display
device.
The DC offset voltage shift circuit 390 for device HP HFBR 2404 is
not required for optical transmission links that do not transmit DC
waveforms or that employ an analog-to-digital converter at the
output of the optical receiver device chip. AC coupling for digital
signals at a rate greater than 0.1 Hz may be used. The light source
LED 310 and light detecting photodiode-amplifier 395 in such
instance, carry digital data which is not affected by the nonlinear
LED transmission characteristics. The problem with such an
approach, however, is that it masks changes in the sensed voltage
waveforms that occur when they become susceptible to EMI. Digital
logic circuits tend to ignore device under test waveform changes
that indicate the onset of EMI susceptibility until the changes
becomes sufficiently large to be catastrophic. Thus, digital logic
circuits do not readily identify the onset of EMI
susceptibility.
A transmitter and a compatible receiver for a digital channel
voltage probe transmission link may be adapted by a person of
ordinary skill in the art from FIGS. 3, 3A and 3B, and by referring
to U.S. Pat. No. 4,937,446, specifically to FIG. 3 and column 4,
line 39 to column 5, line 3 of that patent, and the digital channel
of the aforementioned ETVL commercial product. The digital channel
converter and receiver circuit architecture should be essentially
the same as the analog transmitter 10 and receiver 60 configuration
indicated in FIGS. 3, 3A and 3B. However, transmitter amplifier A4
is not required for a digital channel and may be omitted or
replaced with a transistor amplifier. With respect to the digital
receiver, a digital to analog converter may be used to reproduce
analog signal waveforms at V.sub.OUT.
The 25 nanosecond rise and fall times of the digital transmission
link switching waveforms are governed by the distributed
capacitance of the voltage probe input conductor nonmetallic
material. Digital channel rise times are 30 nanoseconds. The key to
a high gain bandwidth product is the ability to maintain
distributed capacitance as low as possible. A compromise in
amplifier chip and discrete resistor value vs. current drain is
obtained. This resulted in a 80 milliampere current drain and a
bandwidth from 0.1 Hz to 30 MHz. The digital receiver also is
configured to provide a 50 ohm output impedance in order to
accommodate spectrum analyzer monitoring of monitored waveforms
from device 100.
The HFBR 1404 transmitting LED device and the HFBR 2404 receiver
LED device are well matched for use in the present invention.
A Hewlett Packard 8012B pulse generator having rise and fall times
as short as 5 nanoseconds may be used as a design tool. The digital
transmission link should follow these rise and fall times well
enough to provide a 4-volt signal output with a 4-volt generator
input at 10 MHz. Slight changes in the generator rise and fall
times and height should be easily detectable at the digital channel
output.
The hardened optical receiver circuit 60 is a direct coupled two
stage RF amplifier composed of discrete circuits for maximum
switching rise time and fall times. This is not necessary when in
the presence of far fields generated by antennas that are one
meter, or further, from the device 100 and optical transmission
link. Grounding the transmitter 10 and base 20 prevents near field
(less than 3 meters) capacitive coupling into the transmitter 10
enclosures and interfering with the internal circuitry.
Capacitively coupled RF will penetrate any metal enclosure
regardless of how thick, unless it is connected to a good low
impedance RF ground.
The effect of susceptibility to electromagnetic radiation of a
device under test is seen referring to FIGS. 5A, 5B, 5C and 5D and
using a two-channel transmission link. These figures reflect data
recorded in a screen room enclosure at different intensity levels
of a controlled EMI field and frequencies using a digital
channel.
FIGS. 5A-5D are based on photographs of an oscilloscope displaying
signals monitored by a two channel voltage probe transmission link
of the present invention wherein one voltage probe included a clip
type grabber monitoring a 5 volt DC input pin on a transmitter
microprocessor and the other voltage probe included a clip type
grabber monitoring the output pin of a logic gate in the receiver
module of device 100. The monitored transmitted pulse train in
FIGS. 5A-5D is on the bottom trace. The monitored received pulse
train is on the top trace. The reduced rise time of the received
pulse is due to the effect of harness capacitance.
The waveforms just before susceptibility at 50.68 MHz and a field
intensity level of 45 V/m are shown in FIG. 5A. FIG. 5B illustrates
the effect of raising the field intensity level to 78 V/m and
maintaining the frequency constant. A comparison of FIGS. 5A and 5B
indicates that the controlled radiation field affected the signal
transmission within the device under test. The transmitted pulse
contains modulated RF and the received pulse is widened and
inverted.
FIG. 5C shows the waveforms at 101.8 MHz and below 20 V/m, just
before the onset of susceptibility. It can be seen that the
transmitted pulses are modulating the RF at this point, but not
enough noise is present to affect the received pulses. FIG. 5D
shows the increase in susceptibility that occurred with just a
slight increase in field intensity level, from 20 V/m to 35 V/m, at
that same frequency.
The information illustrated in FIGS. 5A-5D could not have been
obtained by visually monitoring the voltage displays of the device
under test or with conventional current probes used in
susceptibility testing or a single channel commercial ETVL
product.
The 5-volt DC input pin on the device microprocessor transmitter
was decoupled to ground with a suitable EMI filter, and another EMI
filter was placed in series with the microprocessor output lead to
an input signal conditioning circuit in the receiver of the device.
The device under test was thus made EMI compatible with the above
filters. Referring to FIG. 6, which is in the same format as FIGS.
5A-5D, plots representative of a frequency sweep from 20 to over
200 MHz with the E field intensity level at 100 V/m are shown.
These signals indicate that the waveform distortion from the
radiation field had been essentially eliminated. A minor (1%)
increase in the transmitted pulse rate was noted between 191 and
204 MHz.
Referring to FIG. 1, transmitter 10 includes a rechargeable 12-volt
battery 40 which can be switched out when an external battery 80
(FIG. 2) is used. The internal battery is preferably a 1.2 AH type
that provides 10 hours of operation for a digital channel module
and 15 hours of operation for an analog channel module before
recharge is required.
Referring to FIG. 2, an external battery module 80, coupled to
transmitter 10 with TWINAX cables also can be used to provide power
for the plurality of transmitters 10 when extended periods of use
between recharge are required. TWINAX cables are available from
Belden Wire and Cable. Preferably, a +12 volt 4.8 amp hour
rechargeable battery pack located within the radiation hardened
base 20 is used to provide power to six transmitters 10. Suitable
shielded leads and EMI filters may be supplied in series with base
20 and the input power terminals of each transmitter 10 to reduce
the possibility of RF (EMI) interference. A larger 60 AH battery
may be used external to base 20 with suitable shielded cables and
radiation hardening precautions.
Each receiver 60 is preferably powered with a 1.2 amp, 24-volt DC
U.S or European compatible wall outlet supply. The supply can
provide power for up to 6 receiver modules.
Each of the optical channel converter circuits 30 and receivers 60
are preferably enclosed in 5 inch high, by 1.5 inch wide, by six
inch deep, EMI hardened circuit, modular metal cases. Internal
module circuitry is electrically and physically isolated from the
metal enclosure. Isolated printed circuit boards with ground planes
prevent RF currents on the enclosure exterior from entering the
transmitter interior or circuitry. Particular attention is paid to
package seams in order to prevent interior enclosure resonances at
RF frequencies having wavelengths that approach the seam
dimensions.
EMI power filters 340 are also provided within each transmitter
module 10 (see FIG. 3), as an additional measure, to prevent RF
from entering the transmitter circuitry through the shielded power
leads. BNC connectors that mate the nonmetallic test probes with
the transmitter provide additional EMI immunity.
Screws located on both the individual optical transmitter modules
and module rack allow them to be grounded to a copper table top or
other RF ground point when in the presence of near fields generated
by strip line fixtures or TEM cells. Shielded power leads and EMI
filters at the power input of each transmitter module may be
required in order to provide maximum transmitter immunity to RF
fields as high as 200 V/m between 10 kHz and 18 GHz.
It should be understood that the transmitter configurations that
are disclosed in U.S. Pat. No. 4,939,446, which uses three probe
connectors to monitor a differential voltage at one test point, and
a configuration that uses only two probe connectors for monitoring
single ended outputs of device 100, also may be used in the
multi-channel embodiment of the present invention.
One skilled in the art will appreciate that the present invention
can be practiced by other than the described embodiments which are
presented for purposes of illustration and not of limitation.
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